The present invention relates to an optical isolator for preventing the reflection of the light rays emitted from a light source at an end face of an optical system and the returning back of the reflected light rays to the light source as well as a method for preparing the same.
When light rays from a light source are transmitted through an optical system, a part of the light rays is reflected on the end face of the optical system and returned back to the light source. For instance, when an optical signal is transmitted through an optical fiber, the light rays emitted from a laser light source are projected on the end face of the optical fiber, most of the light rays pass through the optical fiber as the transmitted light rays, but a part thereof is reflected on the surface of lenses and the end face of the optical fiber, thus returned back to the laser light source and again reflected on the end face of the laser source to thus serve as a source of noises.
An optical isolator is in general employed in order to eliminate such noises. The optical isolator in general comprises a first polarizer, a Faraday rotator and a second polarizer which are arranged in this order. The Faraday rotator is accommodated in a hollow cylindrical magnet. These parts are in general assembled with the aid of an adhesive.
However, an optical isolator which is assembled with the aid of an adhesive suffers from problems in that it may cause outgassing due to the rise in the environmental temperature and that the optical axis thereof is deflected due to the expansion of the adhesive and, therefore, the optical properties thereof are unstable. Moreover, it takes a long time period for hardening the adhesive during preparing the same and correspondingly the production efficiency is low.
It has been tried to solve these problems by assembling the optical isolator with the aid of a solder material. For instance, Japanese Patent Unexamined Publication (hereunder referred to as "J. P. KOKAI") No. Hei 1-200223 discloses an optical isolator which is prepared by forming a metal film capable of being soldered on the portion of each part through which the parts are assembled and then bonded and integrated into an assembly through soldering. According to this patent, gold plating is applied onto the surface portion of each part, i.e., the first polarizer, the Faraday rotator or the second polarizer other than the optical face thereof and they are bonded to one another through the plated areas by soldering.
Such connection of these parts through soldering is more stable than that obtained with the aid of an adhesive and can provide optical isolators having stable properties over a long time period, but the adhesion between the gold plating layer and each part is very low and accordingly the plating layer is often peeled off after the plating or soldering operation. In particular, a glassy layer such as an anti-reflection coating is often applied onto the surface of the parts such as the polarizer and the Faraday rotator and, in this case, the gold plating layer is easily peeled off. In addition, the production cost thereof increases since complicated processes such as plating process are required.